OED says Of obscure origin.

looking around a bit more, I was rewarded with this sidebar on the noun form of slather:

Daring hypothesis: English words for “mud”, “near-mud”, “dried-mud”, and “mud like” outnumber Eskimo words for “snow”, “near-snow”, etc. etc.

Can this be true? Let’s have a look! (GJV)
Sludge, muck, slush, rile, slut, slime, bog, stabble (mud made by footprints), marsh, swill, ooze, slip, morass, slunk (a muddy or marshy place), mere, pulk, swamp, cay, blash, baygall, quag, quagmire, sump, slosh, sludge, squash, wichert (white, chalky mud, Bucks.), sleech (mud deposited by a river), clart, fen, humus, slough, bauger, slabber, warp (a moist bed of alluvial sediment), mush, chaitia (dried mud), gumbo, slumgullion (a muddy deposit in a mining sluice, US), slop, wallow, squad (dial., soft mud), slurry (thin mud or cement), parafango (a mixture of mud and paraffin), sludder, stodge (thick, tenacious mud), quicksand, schlich, slake (mud left by the tide), soss (a slop, Sc), cloam (potter’s mud), sinkhole, gunk, goo, clay, slob[Ir], palus, mire, slather (thin mud, Yorks.), sewerage (street mud), adobe (dried mud), limus, silt, loam, smirch, clag (mud entangled with wool on sheep), and this is only a start. Source: OED2